As we've told you ... the alleged killer of UnitedHealthcare CEO Brian Thompson was apparently active on Goodreads -- a website where people log what they read and what they plan to read.

An account bearing his name logged hundreds of titles over the last 8 years ... and included many more on the "Want to Read" section -- including "Adult Children of Emotionally Immature Parents: How to Heal From Distant, Rejecting, or Self-Involved Parents" by clinical psychologist Lindsay Gibson.

Gibson's book "exposes the destructive nature of parents who are emotionally immature or unavailable. You will see how these parents create a sense of neglect, and discover ways to heal from the pain and confusion caused by your childhood. By freeing yourself from your parents’ emotional immaturity, you can recover your true nature, control how you react to them, and avoid disappointment" according to the Amazon summary of the book.

It's received rave reviews online ... holding a 4.4 out of 5 on Goodreads out of more than 65K reviews.

Of course, Mangione's book tastes appear to be wide-ranging ... so, it's unclear if he wanted to read this book to navigate his own relationships, or if his interest was purely academic.

Mangione holds a master's degree in engineering -- and, this book obviously falls well outside that purview ... so, it's certainly possible this really is providing a window into Mangione's familial life.

As we've said ... other books on Mangione's Goodreads including Unabomber Ted Kaczynski's manifesto which he called a "political revolutionary."

Mangione's family released a statement after he was charged with murder earlier this week ... writing they could not comment on Luigi's arrest as they'd only see what the media had reported.